Why do the unsubscribe “footers” appear as attachments for some recipients?

Mailman automatically appends a “footer” to each message, which includes the unsubscribe instructions.

If your list has “content filtering” disabled, which allows sending attachments or sending HTML-formatted messages to the list, and the messages you send to the list aren’t plain text, the footer can appear as an attachment to the message instead of appearing within the text of it.

This happens because Mailman has no way of knowing where the visual “bottom” of a message is if it isn’t plain text. It’s the subject of a Mailman FAQ page that has more details.

How can I fix this?

One solution, if you’re the only person sending to the list, is to send only plain text messages (not HTML-formatted messages) from your email program.

More generally, if you don’t need to send attachments or HTML-formatted messages to your list, you can turn on “content filtering” in Mailman to fix this. The content filtering strips out everything except the plain text of messages.